On 7/18/23 21:06, HAO CHEN GUI via Gcc-patches wrote:
Hi,
The shift mode will be widen in combine pass if the operand has a normal
subreg. But when the target already has rotate/mask/insert instructions on
the narrow mode, it's unnecessary to widen the mode for lshiftrt. As
the lshiftrt is commonly converted to rotate/mask insn, the widen mode
blocks it to be further combined to rotate/mask/insert insn. The PR93738
shows the case.
The lshiftrt:SI (subreg:SI (reg:DI)) is converted to
subreg:SI (lshiftrt:DI (reg:DI)) and fails to match rotate/mask pattern.
Trying 13, 10 -> 14:
13: r127:SI=r125:SI&0xfffffffffffff0ff
REG_DEAD r125:SI
10: r124:SI=r129:DI#4 0>>0xc&0xf00
REG_DEAD r129:DI
14: r128:SI=r127:SI|r124:SI
Failed to match this instruction:
(set (reg:SI 128)
(ior:SI (and:SI (reg:SI 125 [+-2 ])
(const_int -3841 [0xfffffffffffff0ff]))
(and:SI (subreg:SI (zero_extract:DI (reg:DI 129)
(const_int 32 [0x20])
(const_int 20 [0x14])) 4)
(const_int 3840 [0xf00]))))
Failed to match this instruction:
(set (reg:SI 128)
(ior:SI (and:SI (reg:SI 125 [+-2 ])
(const_int -3841 [0xfffffffffffff0ff]))
(and:SI (subreg:SI (and:DI (lshiftrt:DI (reg:DI 129)
(const_int 12 [0xc]))
(const_int 4294967295 [0xffffffff])) 4)
(const_int 3840 [0xf00]))))
If not widen the shift mode, it can be combined to rotate/mask/insert insn
as expected.
Trying 13, 10 -> 14:
13: r127:SI=r125:SI&0xfffffffffffff0ff
REG_DEAD r125:SI
10: r124:SI=r129:DI#4 0>>0xc&0xf00
REG_DEAD r129:DI
14: r128:SI=r127:SI|r124:SI
REG_DEAD r127:SI
REG_DEAD r124:SI
Successfully matched this instruction:
(set (reg:SI 128)
(ior:SI (and:SI (reg:SI 125 [+-2 ])
(const_int -3841 [0xfffffffffffff0ff]))
(and:SI (lshiftrt:SI (subreg:SI (reg:DI 129) 4)
(const_int 12 [0xc]))
(const_int 3840 [0xf00]))))
This patch adds a target hook to indicate if rotate/mask instructions are
supported on certain mode. If it's true, widen lshiftrt mode is skipped
and shift is done on original mode.
The patch fixes the regression of other rs6000 test cases. They're listed
in the second patch.
The patch passed regression test on Power Linux and x86 platforms.
Thanks
Gui Haochen
ChangeLog
combine: Not winden shift mode when target has rotate/mask instruction on
original mode
To winden shift mode is unnecessary when target already has rotate/mask
instuctions on the original mode. It might blocks the further combine
optimization on the original mode. For instance, further combine the insns
to a rotate/mask/insert instruction on the original mode.
This patch adds a hook to indicate if a target supports rotate/mask
instructions on the certain mode. If it returns true, the widen shift
mode will be skipped on lshiftrt.
gcc/
PR target/93738
* combine.cc (try_widen_shift_mode): Skip to widen mode for lshiftrt
when the target has rotate/mask instructions on original mode.
* doc/tm.texi: Regenerate.
* doc/tm.texi.in (TARGET_HAVE_ROTATE_AND_MASK): Add.
* target.def (have_rotate_and_mask): New target hook.
* targhooks.cc (default_have_rotate_and_mask): New function.
* targhooks.h (default_have_rotate_and_mask): Declare.
Wouldn't it make more sense to just try rotate/mask in the original mode
before trying a shift in a widened mode? I'm not sure why we need a
target hook here.
jeff
